Interpretation of Recent Antarctic Sea Ice Loss: The Key Role of the Southern Annular Mode during the Seasonal Sea Ice Maximum

We provide novel observational insights into the role of the Southern annular mode (SAM) in driving variations in Antarctic sea ice area (SIA). The influence of the SAM on Antarctic SIA exhibits more pronounced seasonality than that indicated in previous work. At the time of the sea ice minimum, positive anomalies in the SAM - and thus enhanced westerlies over the high latitude Southern Ocean - lead to increased SIA over the following four months. In contrast, at the time of the sea ice maximum, positive anomalies in the SAM lead to decreased SIA over the following twelve months. Differences in the sign of the SAM-SIA relationship from one season to the next lead to weak contemporaneous relationships between the SAM and SIA in the annual-mean; differences in the persistence from one season to the next lead to an outsized impact of variations in the SAM during the sea-ice maximum on annual-mean SIA. The roughly 12-month persistence of SAM-induced sea ice anomalies following the sea-ice maximum is consistent with both anomalous shortwave radiative feedbacks and heat storage in the upper ocean. It is argued that a notable fraction of the dramatic losses in annual-mean SIA over the past decade can be traced to unprecedented variations in the SAM during the time of the sea-ice maximum.